1. Field of the Invention
The present invention relates to a development unit comprising a latent-electrostatic-image-bearable member, a development member for supporting a toner thereon and developing a latent electrostatic image formed on the above latent-electrostatic-image-bearable member into a visible toner image with a toner, and a toner layer regulating member for regulating the thickness of a toner layer formed on the above development member.
2. Discussion of Background
Conventionally, to develop a latent electrostatic image formed on a latent-electrostatic-image-bearable member into a visible toner image with a non-magnetic one-component type toner, the toner is distributed to a development member and formed into a thin film thereon by a blade- or roller-shaped toner layer regulating member. When the thin film of the toner formed on the surface of the development member is brought into contact with the latent electrostatic image formed on the latent-electrostatic-image-bearable member, such as a photoconductor, the toner is electrostatically attracted to the latent electrostatic image, so that the latent electrostatic image is developed into a visible toner image with the non-magnetic one-component type toner.
In the above development process, the following steps are repeated on the development member:
1. The toner is supplied onto the development member.
2. The toner on the development member is charged to a predetermined polarity.
3. The toner is formed into a thin film on the development member.
4. The toner is transported to the photoconductor by the development member.
5. The toner formed on the development member is attracted to the latent electrostatic image on the photoconductor to achieve the development.
6. The residual electric charge on the development member is removed.
When the development member has finished one cycle of the development process and moves onto the next cycle, a charge with a counter polarity to that of the toner particles is generated in the development member due to the triboelectric effect with the toner particles. In addition, the electric charges may remain on the surface of the development member and inside thereof because it is easily affected by the residual electric potential on the photoconductor. The above-mentioned electric charges cause the deterioration of the chargeability of toner particles and the development characteristics. As a result, the so-called residual image development occurs.
To solve the above-mentioned problem, various methods for quenching the development member are proposed. For example, there is proposed a quenching method of bringing a member with a low resistivity, such as a metallic brush or an electroconductive sponge into contact with the surface of the development member.
However, when the metallic brush is used for quenching the development member, stripes appear on the obtained images because the metallic brush sweeps away the residual electric charge on the surface of the development member, leaving trails. In the case where the electroconductive sponge is used for quenching the development member, satisfactory results cannot also be obtained. In addition, since the above-mentioned metallic brush or electroconductive sponge is brought into contact with the development member through a thin film of the toner formed on the outer surface of the development member, the sufficient quenching effect cannot be desired.
To improve the quenching effect by the metallic brush, the material and the brush density are studied. The effect by the electroconductive sponge is also pursued with a stress laid upon the number of the sponge rollers. In spite of the aforementioned intensive studies, the residual images cannot completely be cleared from the development member.
Many developments are made on the configuration of the development member in order to obtain a development member of which outer surface can be easily initialized, with the residual electric charges not remained thereon.
For example, a development member is proposed, which comprises an electroconductive coating layer which comprises an elastic material with a volume resistivity of 10.sup.5 to 10.sup.11 .OMEGA..cm, and a semiconductive coating layer which comprises a flexible synthetic resin and electroconductive particles. As the flexible synthetic resin for the semiconductive coating layer of the above development member, acrylonitrile-butadiene rubber, epichlorohydrin rubber, chloroprene rubber or urethane rubber is used, so that it is difficult to make a development member which has a uniform properties. In particular, the resistance of the electrical characteristics of the thus obtained development member to the environmental conditions, such as heat and humidity, is poor. Therefore, the development performance of this development member varies depending on the environmental conditions.
Another development member is proposed. This development member is constructed in such a fashion that a resin or rubber layer with a middle to high volume resistivity and an electrode layer are successively overlaid on an electroconductive elastic substrate with a volume resistivity of 10.sup.6 .OMEGA..cm or less. The environmental resistance of this development member can be remarkably improved by adding to each layer the resin or rubber with the improved heat and humidity resistance, such as silicone resin, silicone rubber, fluoroplastic and fluororubber.
As previously mentioned, however, since the above development member comprises a resin or rubber layer with a middle to high volume resistivity, the electric charges generated by the triboelectric friction between the development member and toner particles gradually accumulate on the surface of the development member. These residual electric charges have an adverse influence on the chargeability of toner particles and the development characteristics at the succeeding development process, and the so-called residual image development takes place.
Some trials are made to leak the residual electric charge from the above development member. For example, electroconductive particles such as carbon black are dispersed in the aforementioned electrode layer of the development member to decrease the resistivity thereof. However, in the case where the electrode layer of the development member becomes electroconductive to such a degree that the residual electric charge can be leaked therefrom, the following problems occur:
(1) The charge quantity of the toner is decreased, which interrupts the transport performance of the toner to the latent-electrostatic-image-bearable member.
(2) As the resistivity of the electrode layer of the development member decreases, the strength of the electric field between the development member and the latent-electrostatic-image-bearable member increases. This causes an extremely high image density and low resolution of images.
(3) When a large amount of the electroconductive particles are added to the electrode layer of the development member, the releasability of the toner particles from the development member is decreased, which induces the so-called toner-filming phenomenon. This decreases the wear-resistance of the development member.
As the method for controlling the chargeability of the toner particles, studies are made in terms of the above-mentioned blade- or roller-shaped toner layer regulating member.
When the toner particles are distributed to the surface of the development member and formed into a thin film thereon by the toner layer regulating member, the toner particles are charged by the friction with the toner layer regulating member and the development member. When the above-mentioned blade- or roller-shaped toner layer regulating member is made of metal, this metallic toner layer regulating member cannot provide the toner particles with the electric charge in an adequate amount by the triboelectric effect.
To solve the above-mentioned problem, a bias is applied to the metallic toner layer regulating member to impart the desired electric charge to the toner particles. This method is capable of controlling the charge quantity of the toner. However, the releasability of the toner from the metallic toner layer regulating member is not so good that the toner adheres to the surface of the toner layer regulating member. Therefore, the adequate electric charge cannot constantly be imparted to the toner and the thin-film of the toner cannot be uniformly formed around the development member for an extended period of time.
To remove the above-mentioned shortcoming, the particular polymers and fluoroplastics with excellent releasability are proposed to employ for the toner layer regulating member as disclosed in Japanese Laid-Open Patent Application 63-81376. This prevents the adhesion of the toner to the toner layer regulating member, and at the same time, the electric charge can sufficiently be imparted to the toner particles.
However, the above polymers with the excellent releasability have insulation properties. Therefore, when the toner particles are charged to a predetermined polarity by the triboelectric effect, the electric charge with a counter polarity to that of the toner particles is generated in the the tone layer regulating member and it easily remains therein. This residual electric charge in the toner regulating member gives rise to the following problems:
(a) A lot of time is required to attain to the charge quantity saturation point. In other words, the rising performance of the toner is degraded with regard to the electric charging.
(b) The charge quantity saturation point of the toner gradually decreases while the copying operation is continued for an extended period of time.
As can be seen from the above-mentioned problems, the high quality images cannot constantly be obtained for a long time.
To solve the above-mentioned problems, finely-divided particles with a low volume resistivity, such as carbon, are added to the above polymers with the excellent releasability to decrease the volume resistivity of the toner layer regulating member. Thus, the residual electric charge in the toner layer regulating member can be easily leaked therefrom. However, the releasability of the toner from the toner layer regulating member is lowered, so that the toner particles in the melted state are deposited on the surface of the toner layer regulating member.